Conventional fluorescent lamps contain mercury which is vaporized during lamp operation. The mercury vapor atoms efficiently convert electrical energy to ultraviolet radiation with a wavelength of approximately 253.7 nm when the mercury vapor pressure is in the range of approximately 2×10−3 to 2×10−2 Torr (optimally about 6×10−3 Torr). In turn, the ultraviolet radiation is absorbed by a phosphor coating on the interior of the lamp wall and converted to visible light.
The temperature of the coldest spot on the inner wall of the lamp when the lamp is operating is referred to as the “cold spot temperature.” The cold spot temperature determines the mercury vapor pressure within the lamp. When a lamp containing only mercury operates with a cold spot temperature above about 40° C., the mercury vapor pressure will exceed the optimal value of 6×10−3 Torr. As the temperature increases, the mercury vapor pressure increases and more of the ultraviolet radiation is self-absorbed by the mercury, thereby lowering the efficiency of the lamp and reducing its light output.
The mercury vapor pressure is maintained within the desired range either by controlling the cold spot temperature of the lamp (“temperature control”) or by introducing other metallic elements into the lamp in the form of amalgams that maintain the mercury vapor pressure (“amalgam control”). Temperature-controlled fluorescent lamps generally operate with a cold spot temperature below about 75° C. (typically ranging from 20-75° C.) and preferably 40-60° C. Such lamps are generally referred to as “low temperature” fluorescent lamps.
Fluorescent lamps with cold spot temperatures above about 75° C. (including, but not limited to, certain types of small diameter, low wattage fluorescent lamps generally known as compact fluorescents) are amalgam-controlled in that they typically require two or more elements in addition to mercury which may be introduced into the lamp as solid ternary or multi-component amalgams. Such amalgam-controlled lamps rely on establishment of thermodynamic equilibrium for proper lamp operation (for example, see U.S. Pat. No. 4,145,634).
Conventional fluorescent lamps are dosed with liquid mercury or zinc-mercury amalgam. The mercury vapor pressure is adjusted by controlling the temperature of the lamps. The mercury in lamps containing a zinc-mercury amalgam is in a metastable, non-equilibrium state, in contrast to the condition predicted by an equilibrium phase diagram.
U.S. Pat. Nos. 5,882,237, 6,339,287, and 6,791,254, each incorporated herein by reference, disclose materials, methods, and lamps containing a binary zinc-mercury amalgam. Binary zinc-mercury amalgam pellets provide a solid mercury dose for temperature controlled fluorescent lamps. They eliminate excessive amounts of liquid mercury and are easily handled at temperatures below 40° C. They also provide methods of dosing a fluorescent lamp with mercury, providing accurate and reliable dosing of fluorescent lamps.
The disclosed prior art pellets are in a metastable non-equilibrium state. They have a zinc-rich outer portion and regions of mercury-rich amalgam in the central regions of the pellet. The saturated zinc amalgam provides a mercury vapor pressure that is approximately 95 percent of the vapor pressure of pure mercury.
However, binary zinc-mercury amalgams had several features that were not as desirable as expected. For example, the zinc-mercury amalgam pellets were often times spheroidal, but not substantially spherical. For example, conventional spheroidal pellets have numerous flat spots and high eccentricity (ratio of average major axis over average minor axis significantly greater than unity). The spheroidal pellets required more processing steps than substantially spherical pellets.
Recently, a zinc-tin-mercury amalgam has been developed that is rounder than binary zinc-mercury amalgams. Although the zinc-tin-mercury amalgam improves upon the shape of binary zinc-mercury amalgam, they have the disadvantage of being sensitive to heat and becoming self-agglomerating.
Binary zinc-mercury amalgam pellets also have the disadvantage of re-absorbing small amounts of mercury over a period of weeks or months. Normally the re-absorption of mercury is not harmful to the operation of the fluorescent lamp. However, it is desirable in industry that the re-absorption of mercury be minimized or eliminated.
Accordingly, there is a need in industry for technological solutions providing materials, devices, and methods to address concerns such as mercury re-absorption and amalgam pellet shape.